The etiology and prevention of blood-borne viral infections in injection drug users (IDUs) have not been fully characterized, and many questions remain regarding which injection practices may result in I infection. Viral hepatitis infections in IDUs are among the most frequently occurring 81ood-bornel l infections in humans; in low HIV-prevalence settings, morbidity and mortality in IDUs attributable to l hepatitis virus infections may exceed that for HIV. Hepatitis C virus (HCV) is highly prevalent in IDU- populations, and is more efficiently transmitted l abouty injection than HIV. Because sexual HCV transmission is relatively rare, it may serve as a highly sensitive biologic marker of direct percutaneous exposure to these infections in drug injectors, and may contribute to understanding the mechanism of transmission of other infections via injection practices. The long-term goal of our research is to advance knowledge of the epidemiology, etiology and-prevention of HIV and hepatitis infections in IDUs. We propose studies that will make new contributions toward our long-term goal: Aim 1. Examine the extent to which HCV prevention education at the Seattle needle exchange program has reduced the risk of HCV infection. Aim 2. Measure the risk of HCV seroconversion associated with specific injection risk behaviors, and calculate the risk of HCV attributable to these practices in the IDU-population. Aim 3. Compare reporting of socially-undesirable or stigmatized injection and sexual risk behavior collected by A-CASI to interviewer-administered data collection methods. Aim 4. Assess the feasibility and disease control benefits of HBV and HCV partner notification for IDUs. Aim 5. Study whether changes in hepatitis C reporting laws are associated with increased reporting in IDUs. The significance of this research is its potential contribution to our understanding of the etiology of these l infections, and examines many practical questions related to the effectiveness of public health surveillance and prevention programs.